Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to electronics and, more particularly, to synchronous rectifier drive circuits.
2. Description of the Background
DC-to-DC power converters are power processing circuits which convert an unregulated input DC voltage to a regulated DC output voltage. Switched-mode DC-to-DC power converters typically include an inverter, a transformer having a primary winding coupled to the inverter, and a rectifying circuit coupled to a secondary winding of the transformer. The inverter typically includes a switching device, such as a field effect transistor (FET), that converts the DC input voltage to an alternating voltage, which is magnetically coupled from the primary winding of the transformer to the secondary winding. The rectifying circuit rectifies the alternating voltage on the secondary winding to generate a desired DC output voltage.
It is known to use synchronous rectifiers (SRs) employing metal-oxide-semiconductor field effect transistors (MOSFETs) to convert the alternating voltage of the secondary winding to the unipolar DC output voltage. The advantage of synchronous rectification is that the forward voltage drop, and hence the power loss, across a MOSFET SR is much less than that of diode devices used in the rectifying circuit. Such SR circuits, however, typically require gate drive circuitry to render the MOSFET at a low resistance during forward conduction and, more importantly, to render it non-conductive during reverse bias. This is because, unlike a diode, a SR may be conductive in both directions (i.e., forward and reverse). Thus, if not properly controlled, reverse current can flow through a MOSFET SR, thereby negatively affecting the efficiency of the power converter.
One known technique to control the gate drive of a MOSFET SR is to couple the alternating voltage from the secondary winding of the transformer to the gate terminal of the MOSFET SR to thereby turn the device on and off in response to the voltage across the secondary winding. This scheme is commonly referred to as xe2x80x9cself-driven synchronous rectification.xe2x80x9d Although usually effective, it is possible that when the voltage on the secondary winding reverses and the gate terminal of the SR is driven off, a delay in turn-off of the SR will provide a period of reverse current in the SR. This has a deleterious xe2x80x9cshortingxe2x80x9d effect on the secondary winding which may limit the turn off voltage and further delay commutation of the SR. Additionally, it is difficult to generate the proper on-state SR bias level in the self-driven configuration.
One known technique to overcome the shortcomings of self-driven synchronous rectifiers is to employ a gate drive circuit coupled to the control terminal of the synchronous rectifier. Control-driven gate drive circuits, however, are complicated to implement. In addition, it is difficult to implement a gate drive circuit driven by the alternating voltage of the transformer that is capable of driving two synchronous rectifiers of a dual output power converter or provide the proper bias levels in low voltage output converters.
Accordingly, there exists a need in the prior art for a manner in which to reduce, and even obviate, the delay in turn-off of a SR, to thereby minimize, or eliminate, any period of reverse conduction of the SR and the subsequent shorting effect. There further exists a need for a gate drive circuit that can efficiently and effectively drive the synchronous rectifiers for a dual output converter. Still further there exists a need for a gate drive circuit that is capable of providing the required SR bias level in low output converters.
According to one embodiment, the present invention is directed to a dual output power converter. The dual output power converter includes a transformer having first and second secondary windings, a first synchronous rectifier coupled to the first secondary winding of the transformer for converting an alternating voltage at the first secondary winding to a first DC output voltage, a second synchronous rectifier coupled to the second secondary winding of the transformer for converting an alternating voltage at the second secondary winding to a second DC output voltage, and a drive circuit coupled to each of the first and second synchronous rectifiers for turning on the first and second synchronous rectifiers when the alternating voltage at the first and second secondary windings transition from a first polarity to a second polarity, and for turning off the first and second synchronous rectifiers when the alternating voltage at the first and second secondary windings transition from the second polarity to the first polarity.
According to another embodiment, the present invention is directed to a drive circuit for a synchronous rectifier, wherein the synchronous rectifier is for converting an alternating voltage to a DC voltage. The drive circuit includes a first switch for supplying a drive current to a control terminal of the synchronous rectifier when the when the alternating voltage is at a first polarity, a second switch for shunting the drive current from the control terminal of the synchronous rectifier when the alternating voltage transitions from the first polarity to a second polarity, a pulse transformer having a primary winding and a secondary winding, wherein the primary winding is responsive to a condition causing transitions of the alternating voltage between the first and second polarities, and wherein conduction of the second switch is controlled by a voltage across the secondary winding of the pulse transformer, and a differentiator circuit coupled to the pulse transformer.
According to another embodiment, the present invention is directed to a power supply with parallel-connected converters having a cross-coupled charge pump arrangement. The power supply includes: a first converter including a first transformer, a first synchronous rectifier for converting an alternating voltage at a secondary winding of the first transformer to a first DC voltage, a first drive circuit coupled to the first synchronous rectifier for turning on the first synchronous rectifier when an alternating voltage at the secondary winding of the first transformer transitions from a first polarity to a second polarity, and a first charge pump coupled to the secondary winding of the first transformer; and a second converter connected in parallel with the first converter, wherein the second converter includes a second transformer a second synchronous rectifier for converting an alternating voltage at a secondary winding of the second transformer to a second DC voltage, a second drive circuit coupled to the second synchronous rectifier for turning on the second synchronous rectifier when an alternating voltage at the secondary winding of the second transformer transitions from a first polarity to a second polarity, and a second charge pump coupled to the secondary winding of the second transformer, wherein the first charge pump is for turning on the second synchronous rectifier when the alternating voltage at the secondary winding of the second transformer transitions from the second polarity to the first polarity, and wherein the second charge pump is for turning on the first synchronous rectifier when the alternating voltage at the secondary winding of the first transformer transitions from the second polarity to the first polarity.
The present invention provides an advantage over prior art self-driven synchronous rectification schemes because it provides a manner for eliminating delay in the turn-off of a synchronous rectifier, thus providing the advantage of eliminating the shorting effect of the secondary winding of the transformer. Embodiments of the present invention also provide the advantage of having a mechanized synchronous rectifier turn-on system operable at, for example, low output voltages. These and other benefits of the present invention will be evident from the detailed description hereinbelow.